This is the counter at my local Borders for "eReaders". There are countless devices, prices, preferences, etc. If you are in the market for one of these devices, would you rather sift through all these options or pick the trusty and ultra-popular Kindle by Amazon? It's a similar situation to Android vs. iPhone. While there is absolutely an advantage in the cross device platforms (including lower prices for consumers) - it also creates headaches. There are so many Android handsets that it creates confusion... and it can create quality issues across the plethora of devices: some are great and some are sub-par.
And the same situation will occur as tablets become more prevelant / popular. Consumers will need to choose between dozens of options (hardware & software) and the iPad (a popular, trusted product that will likely come at a premium).